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		<title>three.js webgl2 - volume - cloud</title>
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			<a href="https://threejs.org" target="_blank" rel="noopener">three.js</a> webgl2 - volume - cloud
		</div>

		<script type="module">
			import * as THREE from '../build/three.module.js';
			import { OrbitControls } from './jsm/controls/OrbitControls.js';
			import { ImprovedNoise } from './jsm/math/ImprovedNoise.js';

			import { GUI } from './jsm/libs/dat.gui.module.js';
			import { WEBGL } from './jsm/WebGL.js';

			if ( WEBGL.isWebGL2Available() === false ) {

				document.body.appendChild( WEBGL.getWebGL2ErrorMessage() );

			}

			const INITIAL_CLOUD_SIZE = 128;

			let renderer, scene, camera;
			let mesh;
			let prevTime = performance.now();
			let cloudTexture = null;

			init();
			animate();

			function generateCloudTexture( size, scaleFactor = 1.0 ) {

				const data = new Uint8Array( size * size * size );
				const scale = scaleFactor * 10.0 / size;

				let i = 0;
				const perlin = new ImprovedNoise();
				const vector = new THREE.Vector3();

				for ( let z = 0; z < size; z ++ ) {

					for ( let y = 0; y < size; y ++ ) {

						for ( let x = 0; x < size; x ++ ) {

							const dist = vector.set( x, y, z ).subScalar( size / 2 ).divideScalar( size ).length();
							const fadingFactor = ( 1.0 - dist ) * ( 1.0 - dist );
							data[ i ] = ( 128 + 128 * perlin.noise( x * scale / 1.5, y * scale, z * scale / 1.5 ) ) * fadingFactor;

							i ++;

						}

					}

				}

				return new THREE.DataTexture3D( data, size, size, size );

			}

			function init() {

				renderer = new THREE.WebGLRenderer();
				renderer.setPixelRatio( window.devicePixelRatio );
				renderer.setSize( window.innerWidth, window.innerHeight );
				document.body.appendChild( renderer.domElement );

				scene = new THREE.Scene();

				camera = new THREE.PerspectiveCamera( 60, window.innerWidth / window.innerHeight, 0.1, 100 );
				camera.position.set( 0, 0, 1.5 );

				new OrbitControls( camera, renderer.domElement );

				// Sky

				const canvas = document.createElement( 'canvas' );
				canvas.width = 1;
				canvas.height = 32;

				const context = canvas.getContext( '2d' );
				const gradient = context.createLinearGradient( 0, 0, 0, 32 );
				gradient.addColorStop( 0.0, '#014a84' );
				gradient.addColorStop( 0.5, '#0561a0' );
				gradient.addColorStop( 1.0, '#437ab6' );
				context.fillStyle = gradient;
				context.fillRect( 0, 0, 1, 32 );

				const sky = new THREE.Mesh(
					new THREE.SphereGeometry( 10 ),
					new THREE.MeshBasicMaterial( { map: new THREE.CanvasTexture( canvas ), side: THREE.BackSide } )
				);
				scene.add( sky );

				// Texture

				const texture = new THREE.DataTexture3D(
					new Uint8Array( INITIAL_CLOUD_SIZE * INITIAL_CLOUD_SIZE * INITIAL_CLOUD_SIZE ).fill( 0 ),
					INITIAL_CLOUD_SIZE,
					INITIAL_CLOUD_SIZE,
					INITIAL_CLOUD_SIZE
				);
				texture.format = THREE.RedFormat;
				texture.minFilter = THREE.LinearFilter;
				texture.magFilter = THREE.LinearFilter;
				texture.unpackAlignment = 1;

				cloudTexture = texture;

				// Material

				const vertexShader = /* glsl */`
					in vec3 position;

					uniform mat4 modelMatrix;
					uniform mat4 modelViewMatrix;
					uniform mat4 projectionMatrix;
					uniform vec3 cameraPos;

					out vec3 vOrigin;
					out vec3 vDirection;

					void main() {
						vec4 mvPosition = modelViewMatrix * vec4( position, 1.0 );

						vOrigin = vec3( inverse( modelMatrix ) * vec4( cameraPos, 1.0 ) ).xyz;
						vDirection = position - vOrigin;

						gl_Position = projectionMatrix * mvPosition;
					}
				`;

				const fragmentShader = /* glsl */`
					precision highp float;
					precision highp sampler3D;

					uniform mat4 modelViewMatrix;
					uniform mat4 projectionMatrix;

					in vec3 vOrigin;
					in vec3 vDirection;

					out vec4 color;

					uniform vec3 base;
					uniform sampler3D map;

					uniform float threshold;
					uniform float range;
					uniform float opacity;
					uniform float steps;
					uniform float frame;

					uint wang_hash(uint seed)
					{
							seed = (seed ^ 61u) ^ (seed >> 16u);
							seed *= 9u;
							seed = seed ^ (seed >> 4u);
							seed *= 0x27d4eb2du;
							seed = seed ^ (seed >> 15u);
							return seed;
					}

					float randomFloat(inout uint seed)
					{
							return float(wang_hash(seed)) / 4294967296.;
					}

					vec2 hitBox( vec3 orig, vec3 dir ) {
						const vec3 box_min = vec3( - 0.5 );
						const vec3 box_max = vec3( 0.5 );
						vec3 inv_dir = 1.0 / dir;
						vec3 tmin_tmp = ( box_min - orig ) * inv_dir;
						vec3 tmax_tmp = ( box_max - orig ) * inv_dir;
						vec3 tmin = min( tmin_tmp, tmax_tmp );
						vec3 tmax = max( tmin_tmp, tmax_tmp );
						float t0 = max( tmin.x, max( tmin.y, tmin.z ) );
						float t1 = min( tmax.x, min( tmax.y, tmax.z ) );
						return vec2( t0, t1 );
					}

					float sample1( vec3 p ) {
						return texture( map, p ).r;
					}

					float shading( vec3 coord ) {
						float step = 0.01;
						return sample1( coord + vec3( - step ) ) - sample1( coord + vec3( step ) );
					}

					void main(){
						vec3 rayDir = normalize( vDirection );
						vec2 bounds = hitBox( vOrigin, rayDir );

						if ( bounds.x > bounds.y ) discard;

						bounds.x = max( bounds.x, 0.0 );

						vec3 p = vOrigin + bounds.x * rayDir;
						vec3 inc = 1.0 / abs( rayDir );
						float delta = min( inc.x, min( inc.y, inc.z ) );
						delta /= steps;

						// Jitter

						// Nice little seed from
						// https://blog.demofox.org/2020/05/25/casual-shadertoy-path-tracing-1-basic-camera-diffuse-emissive/
						uint seed = uint( gl_FragCoord.x ) * uint( 1973 ) + uint( gl_FragCoord.y ) * uint( 9277 ) + uint( frame ) * uint( 26699 );
						vec3 size = vec3( textureSize( map, 0 ) );
						float randNum = randomFloat( seed ) * 2.0 - 1.0;
						p += rayDir * randNum * ( 1.0 / size );

						//

						vec4 ac = vec4( base, 0.0 );

						for ( float t = bounds.x; t < bounds.y; t += delta ) {

							float d = sample1( p + 0.5 );

							d = smoothstep( threshold - range, threshold + range, d ) * opacity;

							float col = shading( p + 0.5 ) * 3.0 + ( ( p.x + p.y ) * 0.25 ) + 0.2;

							ac.rgb += ( 1.0 - ac.a ) * d * col;

							ac.a += ( 1.0 - ac.a ) * d;

							if ( ac.a >= 0.95 ) break;

							p += rayDir * delta;

						}

						color = ac;

						if ( color.a == 0.0 ) discard;

					}
				`;

				const geometry = new THREE.BoxGeometry( 1, 1, 1 );
				const material = new THREE.RawShaderMaterial( {
					glslVersion: THREE.GLSL3,
					uniforms: {
						base: { value: new THREE.Color( 0x798aa0 ) },
						map: { value: texture },
						cameraPos: { value: new THREE.Vector3() },
						threshold: { value: 0.25 },
						opacity: { value: 0.25 },
						range: { value: 0.1 },
						steps: { value: 100 },
						frame: { value: 0 }
					},
					vertexShader,
					fragmentShader,
					side: THREE.BackSide,
					transparent: true
				} );

				mesh = new THREE.Mesh( geometry, material );
				scene.add( mesh );

				//

				const parameters = {
					threshold: 0.25,
					opacity: 0.25,
					range: 0.1,
					steps: 100
				};

				function update() {

					material.uniforms.threshold.value = parameters.threshold;
					material.uniforms.opacity.value = parameters.opacity;
					material.uniforms.range.value = parameters.range;
					material.uniforms.steps.value = parameters.steps;

				}

				const gui = new GUI();
				gui.add( parameters, 'threshold', 0, 1, 0.01 ).onChange( update );
				gui.add( parameters, 'opacity', 0, 1, 0.01 ).onChange( update );
				gui.add( parameters, 'range', 0, 1, 0.01 ).onChange( update );
				gui.add( parameters, 'steps', 0, 200, 1 ).onChange( update );

				window.addEventListener( 'resize', onWindowResize );

			}

			function onWindowResize() {

				camera.aspect = window.innerWidth / window.innerHeight;
				camera.updateProjectionMatrix();

				renderer.setSize( window.innerWidth, window.innerHeight );

			}

			let curr = 0;
			const countPerRow = 4;
			const countPerSlice = countPerRow * countPerRow;
			const sliceCount = 4;
			const totalCount = sliceCount * countPerSlice;
			const margins = 8;

			const perElementPaddedSize = ( INITIAL_CLOUD_SIZE - margins ) / countPerRow;
			const perElementSize = Math.floor( ( INITIAL_CLOUD_SIZE - 1 ) / countPerRow );

			function animate() {

				requestAnimationFrame( animate );

				const time = performance.now();
				if ( time - prevTime > 1500.0 && curr < totalCount ) {

					const position = new THREE.Vector3(
						Math.floor( curr % countPerRow ) * perElementSize + margins * 0.5,
						( Math.floor( ( ( curr % countPerSlice ) / countPerRow ) ) ) * perElementSize + margins * 0.5,
						Math.floor( curr / countPerSlice ) * perElementSize + margins * 0.5
					).floor();

					const maxDimension = perElementPaddedSize - 1;
					const box = new THREE.Box3( new THREE.Vector3( 0, 0, 0 ), new THREE.Vector3( maxDimension, maxDimension, maxDimension ) );
					const scaleFactor = ( Math.random() + 0.5 ) * 0.5;
					const source = generateCloudTexture( perElementPaddedSize, scaleFactor );

					renderer.copyTextureToTexture3D( box, position, source, cloudTexture );

					prevTime = time;

					curr ++;

				}

				mesh.material.uniforms.cameraPos.value.copy( camera.position );
				// mesh.rotation.y = - performance.now() / 7500;

				mesh.material.uniforms.frame.value ++;

				renderer.render( scene, camera );

			}

		</script>

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